Charged beam lithography involves bombarding a resist layer with a scanning charged beam whose intensity varies with time (i.e., a spatially selective scanning beam having a prescribed resulting lateral pattern). The charged beam is typically an electron beam (a "direct-writing electron beam"). The resist layer is located on a workpiece. The workpiece typically comprises a transparent substrate to be used for a phase-shifting mask or a semiconductor wafer to be used for an integrated circuit.
The resist layer is developed after the bombardment, whereby a desired pattern of openings is formed in the resist layer in accordance with the resulting lateral pattern of the charged beam. Then the thus patterned resist layer is used for defining features in the workpiece--i.e., for patterning the workpiece in accordance with the lateral pattern of the beam, using the patterned resist layer as a patterned mask. For example, using a semiconductor wafer as the workpiece, the patterned resist layer is bombarded with ions that are thus implanted into the wafer or that react with the wafer to form indentation ("recess") regions in the wafer. Alternatively, the patterned resist layer is used as a patterned mask for the deposition of patterned metal layers on the wafer, in order to metallize integrated circuits. Using a transparent substrate (with a patterned layer of opaque portions on typically one of its major surfaces) the patterned resist layer is used as a patterned mask for spatially selectively removing portions of the thickness of the transparent substrate, or portions of the thickness of the opaque layer, or of both--i.e., portions of the workpiece. Moreover, the patterned resist layer is typically completely removed after being used as a patterned mask in conjunction with the semiconductor wafer or with the transparent substrate.
A known method for defining a desired pattern in a resist layer involves exposing the resist layer to a direct-writing, scanning electron beam, followed by wet developing the resist layer, whereby the desired patterned resist layer is formed. One problem that arises is called "charging effects"--that is, electronic charges that are introduced by the electron beam at earlier moments of times during the scanning undesirably laterally defect the electron beam at neighboring locations at subsequent moments of time, whereby the resulting pattern of exposure of the resist layer to the electron beam is not the same as was desired.
In a paper authored by Keiko Yano et al., entitled "A New Process for Reducing the Charging Effect in Electron Beam Lithography," and published in JJAP Series 4, Proc. of 1990 Intern. MicroProcess Conference, pp. 147-150 (1990), a method of patterning an electron-beam-sensitive resist layer is taught with desirably reduced charging effects. In that method, a direct-writing (scanning) electron beam is used for producing the desired pattern in the resist layer after the latter has been exposed to the beam and has been wet developed, but with the addition of a grounded electrically conductive organic layer overlying the resist layer during the electron beam bombardment of the resist layer, and with the addition of an intermediate protective layer intervening between the conductive organic layer and the resist layer. The conductive organic layer is transparent to the electron beam, and its purpose is to remove the undesirable charges by conducting them to ground.
The conductive organic layer is formed by spin-coating from a solution of the polymer in an organic solvent. The purpose of the intermediate protective layer is to suppress undesirable global removal of the resist layer by the organic solvent during the spin-coating process. Ibis protective layer can simply be a polyvinyl alcohol (hereinafter "PVA") layer, which can be removed--subsequent to a removal of the conductive organic layer after the electron bombardment has been completed--by means of simply dissolving it in water.
Disadvantages of the foregoing method can arise from a formation and subsequent accumulation of unwanted polymer residues at the interface of the PVA and resist layers. More specifically, the unwanted residues can coat the workpiece and thus render it difficult to transfer the pattern of the patterned resist layer to the workpiece. Disadvantages can also arise from the random localized formation of unwanted pinholes in the resist layer during the removal of the conductive organic layer, owing to the porousness of the PVA layer and to the solubility of the resist in the organic solvent that is used to dissolve the conductive organic layer. More specifically, pinholes in the protective layer enable the solvent, which is subsequently used for removing the conductive organic layer after the electron bombardment has been completed, to penetrate down to, and to dissolve localized portions of, the resist layer. Therefore, alleviation of one or both of these disadvantages is desirable.